The present invention relates to optical fibre-based devices which utilise Raman scattering. In particular, the devices include optical amplifiers, optical modulators and lasers.
The capability of silica optical fibres to carry large amounts of information over long repeaterless spans in telecommunications systems makes lightwave communications very attractive. Numerous channels at different wavelengths can be multiplexed on the same fibre to maximise the use of the available bandwidth. Higher transmission powers and/or lower loss fibres can increase system margins.
A device commonly used to compensate for the losses in fibre is the erbium-doped fibre amplifier (EDFA). This is based on silica fibre doped with erbium, which, when pumped with light of a suitable wavelength, provides optical gain and hence amplification of optical signals passing through the fibre. The properties of erbium mean that the bandwidth of the available gain is approximately 30 to 40 nm around 1550 nm. This places a severe limitation on the bandwidth of the telecommunications system as a whole, because only optical signals within this wavelength range can be adequately transmitted.
Also, attempts to fully utilise the capabilities of silica fibres for telecommunications applications tend to be limited by nonlinear interactions between information-bearing optical signals travelling in the fibre, and the fibre itself. These optical nonlinearities can lead to distortion, excess attenuation and interference of the optical signals, resulting in system degradations. There are many nonlinear optical effects in fused silica fibres, each of which have unique properties. Generally, these nonlinear effects can be divided into two categories. The first relates to elastic phenomena. In an optical fiber the core in which the optical signals travel has a specific refractive index that determines how fast light travels through it. However, depending upon the intensity of light travelling in the core, this refractive index can change. This intensity-dependence of the refractive index is called the Kerr effect. It can cause self-phase modulation (SPM) of a signal, whereby light at one wavelength channel can broaden out spectrally onto adjacent wavelength channels through its own self action within the medium. It can also cause cross-phase modulation (XPM), whereby several different wavelengths in a WDM system can cause each other to spectrally broaden and spread out. Finally, it can result in four-wave mixing (FWM), in which two or more signal wavelengths can interact to create a new wavelength. A second category of nonlinear effects results from stimulated inelastic scattering, in which energy is exchanged between the electromagnetic field and the dielectric medium. Effects in this category are stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS), which are related to the excitation of vibrational modes of silica. The main difference between the two is that optical phonons participate in SRS while acoustic phonons participate in SBS.
Many nonlinear effects can be problematic for telecommunications systems. However, SRS can be used for optical amplification, as an alternative to EDFAs.
The Raman effect is described quantum-mechanically as a scattering of an incident photon by a molecule to a lower frequency photon while at the same time the molecule makes a transition between vibrational modes. SRS occurs when the incident photons are contained in a very intense pump wave. This results in very high Raman gain such that much of the pump energy is transferred to light at the lower frequency, called the Stokes wave. This nonlinear process can turn optical fibers into broadband Raman amplifiers [1], Raman lasers [2] and also SRS-based modulators [3]. On the other hand, it can also limit the performance of multichannel lightwave systems by transferring energy from one channel to red-shifted neighbouring channels.
A Raman amplifier provides an attractive and convenient choice of broadband optical amplifier due to the fact that the bandwidth of the gain can be tuned by simply changing the pump wavelength. The gain can be tailored to a desired communications wavelength, hence overcoming the drawback of the narrow gain bandwidth capabilities of EDFAs. Thus, particular interest in Raman amplifiers lies in the optical communication bands (short (S) band and long (L) band) outside the conventional EDFA gain window (C band). Moreover, by pumping such amplifiers with a combination of different pump wavelengths it is possible to further extend and shape the available gain bandwidth.
A problem with the Raman amplifier is that it requires a high power pump laser. However, the recent availability of high power lasers (typically ˜b 5 W) in the 1400 nm region makes Raman amplification in the desired S. C and L bands possible. A further drawback of the Raman amplifier is the need for long lengths of fibre (˜10 km), which increases the bulk and cost of these devices.
It is thus desired to provide an optical fibre amplifier which addresses the disadvantages of known devices, in particular problems relating to fibre length and limited bandwidth.